Fibrous filter media are used in various types of filter devices to trap large and small particles in liquid and gas streams. Such filter media are typically formed from multiple layers of coarse and fine fibers extending parallel to an upstream face surface of the filter media. An outer layer of coarse fibers forms a bulk filtration layer for filtering of larger particles, while an inner or underlying layer of fine fibers provides filtering of small particles. Fine fibers are often provided in a thin layer laid down on a supporting substrate or used with one or more protective layers to obtain a variety of benefits, including increased efficiency, reduced initial pressure drop, cleanability, reduced filter media thickness, and to provide an impermeable barrier to various liquids, such as water. However, prior approaches have several inherent disadvantages, including the need for a supporting substrate, a risk of delamination of the fine fiber layer from the substrate, more rapid loading of the filter by captured particles, and the alignment of fine fibers parallel to the media face surface.
On the molecular level, fibrous materials also trap contaminants with electrostatic forces, including ionic bonding, hydrogen bonding, and Van der Waals forces. These electrostatic interactions occur on the fiber surface. Because these interactions are known to increase non-linearly at sub-micron length scales, functional improvement in fibrous filter media is largely based on minimizing denier (linear mass density or fiber diameter). Although the production of filter media comprising very fine fibers having a high surface-to-volume ratio, such as microfibers and nanofibers, has recently been emphasized in the industry, processing limitations associated with traditional methods of producing such fibers limit the utility of these materials in filtration applications. For example, extruded microfibers require the use of solvents and, or alternatively, immiscible polymer blends to split fibers to submicron length scales, while production of nanofibers by the electrospinning method requires high-voltage (i.e., kilovolt) electric fields.
Accordingly, what is needed are improvements in filter media and filter devices.